This below walkthrough is NOT complete. It is just the beginning. If you find something please enhance this walkthrough!

You "land" in a great room with a red carpet. Lara must be king! Explore the room. Turn to the left and pickup behind the stars the Automatic Pistol Clips(1),the Shotgun Shells (2) and Uzi clips (3). Now walk to the other stairs and pickup from behind the M16 Clips (4), the Automatic Pistol Clips (5) and the Uzi Clips (6). When you pickup the ammo shoot three dogs (1,2,3). Walk towards the great closed doors and pickup the Grenade Launcher (7). Now take the right stairs as the closed doors are behind you. Pickup the Shotgun Shells (8). Turn around and flip the switch the open the great closed doors. Kill everything that trying to hunt you down. Kill two bad guys (4,5) and a dog (6). Enter the room and kill another two bad guys (7,8). Climb on the table and pickup the Automatic Pistol Clips (9), Automatic Pistol (10) and Grenade Clips (11). Flip the switch near the closed door. The door will open. Kill a bad guy (9) and two rats (10,11). Go the right and kill the two dogs (12,13) that are running up and down the stairs. On your left you can pickup the Uzi Clips (12), Automatic Pistol Clips (13) and the M16 Clips (14). Climb the stairs and kill another two rats (14,15) and a dog (16). Continue up the stairs. Kill two bad guys (17,18) and a flame thrower (19). When you have reached the top of the stairs. Climb up the last block and kill the dog (20) and two bad guys (21,22). Now there is no way up so we must go downstairs again. You will see a switch. It is a timed switch. When you flip it, a door downstairs will. The available space to get to the door is for me to little. So I didn't find out which door was open for a little while. I think the door by the fireplace. So you can give it a try. When you come down (and you hear the door closing) go straight forward and take in the passage the first passage leading towards the room where we started. Kill all the rats (23,24,25,26,27,28,29) that are walking around. Go back the way you came. Go left, left, right and follow the hallway. You will find the passage full with sharp blades. Climb on the block and pickup the Little Medikit (15). Jump from the block and go left. pickup the Harpoons (16), Grenades (17) and Uzi Clips (18). You can climb the stairs but it is a dead end! Head down and make you way towards somewhat raised block. Jump up and climb the ledge. Watch out for two flamethrowers (30,31) Kill them, before they can set you on fire!. Climb up again. walk into the new chamber. Kill the two rats (32,33). Pickup the Uzi Clips (19), the Automatic Pistol Clips (20) and the M16 Clips (21). Walk to the left and kill the bad guy (34). head back and flip the switch. The first door will open. Kill the two bad guys (35,36). The pickup contains the M16 Clips (22), 2 sets of Harpoon Clips (23,24) and a Little Medikit (25). The other door will not open. Go back to the first room and to the fireplace room. Go left and right, up the stairs. You have to throw the timed switch and run downstairs, then left and through the open door. Get in the pool and pick up Harpoons (26) and Automatic Pistol Clips (27). Get out, shoot five rats (37,38,39,40,41) on the stairs and pull the second timed lever. Sprint through the door and jump over the glass. Push both levers in this room and climb out. Get into the left room and shoot two bad guys (42,43) and a flamethrower (44). Get M16 Clips (28) and shoot a dog (45) and a boss (44) in the other part of the room. Pick up 2 Uzi Clips (29,30) and pull the box under the button. Now I found no way further.

NOT the end of the level.
Pickups: 30
Kills: 44
Secrets: 0 until now.

Note by Mman/Semfry:

This second walkthrough was partly finished by DJ Full (up to the detonator key), and I have finished the rest. If anything is still unclear I have a full video walkthrough with comments/help as subtitles. It differs from this walkthrough slightly as I avoid the Grenade Launcher to skip EPIC FIGHT #1 (and avoid unnecessary conflicts in general), but overall it's very similar: https://www.youtube.com/watch?v=CX6P5KFRc9k

EPIC FIGHT #1
To kill first 20 enemies described in previous review ( most of these enemies spawn when you walk on the Grenade Launcher tile, so you can just ignore it and skip this fight):
1. Stand in front of the door and move left till you see a black gunman on the right side of the next room. Aim, jump left to hide behind the wall. The gunman should do circles when running, appearing and hiding behind next wall, so:
1. When he turns back, strafe right to see him and hit him.
2. When he tries to shoot you, strafe left and hide from him.
3. Repeat steps 1. and 2. till he dies.
Now target the black gunman located behind the table and jump backwards until you are beyond his sight. Shoot him from here. Move to the right side of the door and stand on the right square, closest to the open door. Aim left - Lara should kill the first of two white gunmen in the room. Meanwhile, watch out for the flamethrowers - they can get out of the room and follow you, just like the black gunmen can. Get rid of both flamethrowers without entering the room. Now switch to Shotgun, enter the room starting from the right square closest to the open door and start shooting immediately, so you can kill one of the white gunmen at once. Jump over the second, roll in midair and eliminate the next one. Stand on the table and shoot 7 rats on the floor.
WARNING: Watch out for a dog falling down from the ceiling!!
Now press the switch, draw auto pistols and just kill a rat and a black gunman. He turns around quite slowly, so shoot him in his back. Now it's time to kill a dog running towards you from the stairs.
WARNING: If the dog didn't fall down from the ceiling yet, it can do it now!!

THE BLADES
Pick up the ammo under the stairs. Don't go up yet. Go back and enter the passage with lots of cutting blades. Make a running jump, turning left in midair, and jump over all of the knives. You can do it without damage, but watch out - if you hit the wall, you'll probably die. Take a small medikit - YES! Jump over the next set of blades and take the ammo. Go back onto the square where you picked up the medikit.

EPIC FIGHT #2
Change weapon to the shotgun. Climb the next block and kill two flamethrowers. Watch out for the third one in the next room - he can aim down and fry you insantly. Blow him up with a grenade gun.
Pick up the goodies left by killed flame-rs and go up. There are two rats and a black gunman. Don't press the switch yet by continue fight by getting to the second locked door and killing a dog and a black gunman through it. Do it now, so you won't be surprised by these enemies when you return to this place in the future. Now press the switch and pick up the ammo placed around. Go through the open door and be prepared for graphic bug. Kill the black gunman - he is faster than the big one behind him. Now draw the Grenade Launcher and finish the battle.
NOTE: if the dog didn't fall from the sky yet...

RETURN
Pick up everything in the room and go back through the blades. Do it the same way as previously, but now with turning right in midair. Again, if you hit the wall, it's game over, so:
1. Turn back from the screen (I'm talking to YOU, not to Lara).
2. Make a cup of tea.
3. Meanwhile, take a relaxing walk around your room.
4. Drink warm tea that will calm you down.
5. Turn on some sweet music.
6. Take a deep breath.
7. Look through the window at some distant object so your eyes can rest.

Return to game and make the jump.

EPIC FIGHT #3
Go upstairs. Kill another dog and two rats. Now be careful. There is a white gunman up there. Stand two blocks under him and move left as far as you can. Jump up and down and shoot him with normal pistols. The next black gunman can be killed without problem, but when you eliminate him, switch weapon to Grenade Launcher. When you step forwards, a flamethrower will appear. Blow him up. Now get to the highest part of the level and shoot a dog, a black and a white gunmen through the window.
Watch out: they can get to you through the window, so be fast.

TIMED RUN
Press the switch. This run isn't as annoying as it was described in first review. Just remember where to turn. If you hit the wall three times - go back, because you won't make it. After you beat the timed run, STOP IMMEDIATELY when the door shuts behind you.

EPIC FIGHT #4
The third square behind the door triggers another multi-enemy-fight. Encounter 2 rats, 1 dog and 1 white gunman. The rats may draw Lara's attention from the main threat, so for this fight I recommend: switching your weapon to Auto Pistols and eliminating each rat with one precise shot. Now jump forward, do the roll in midair and take out the dog. One more backflip and you should take out the gunman easily with a minimum loss of health.

THE DIVE
Welcome to the sewer section. Jump into the water. Get the harpoon gun, flares and a large medipack from one of the holes (left) and dive deeper into the another (right). There is a lot of goodies down here: ammo for M16, Uzi, Shotgun, Harpoon Gun and a Grenade Gun. And a small medipack.

THE CLIMB
Get out of the water.
1. Climb.
2. Kill a rat.
3. Repeat steps 1. and 2.

THE GLASS TRAP
It is mentioned in previous review. You have to go through both door or you're doomed. The best way is:
1. Pull the switch.
2. Jump left.
3. Roll.
4. Run and jump over the glass.

THE SWITCHES
The right switch lets you out, the left lets you into the another section of level. Start with the left one, because (just like in case of the glass trap) both doors are timed. Don't worry - there is enough time to do this run. But you MUSTN'T stop between the doors!! If you do this for too long, the first door will shut behind you and you will have to go again through the whole part of the level you have just completed!!! When you pass the timed run, there is another epic fight.

EPIC FIGHT #5
Draw Grenade Launcher and kill a white gunman. Do the same with the black gunman. A flamethrower is waiting for you and is able to FRY YOU THROUGH THE SOLID BLOCKS! So be careful when blowing him up. Switch to pistols and move forward. A dog will appear so get rid of it.

THE HALL OF MONOTONY
You're in the fabulous Hall Of Monotony, the place made of hundreds of sacred squares covered in round, golden ornaments. It seems to be the copy of legendary Amber Chamber... But I don't know if it really is. Pick up the M16 and Uzi ammo, and... YES! THE UZIS!!! You now should have 720 bullets for this weapon, so it will be used for mass murdering in the next epic fight.

EPIC FIGHT #6
This is the most weird fight in TR Custom Level I've ever had. It is worth to get so far in this level to do it. Draw Uzis, jump onto the highest section of the room and aim down. I will not tell any more...

THE PUZZLE
Yes! There is a puzzle in this game! The hall is full of pushable blocks, some of which are really unpushable in any direction. Don't mind them. Just move the blocks you can move to make a way to the switch on the wall.

THE MOST ANNOYING TIMED RUN EVER
Note: You need to reach the destination room AND get out of it before the door shuts behind you. If you don't make it - you are doomed. Press the switch. From now on you have 46 seconds to get to the door. You have seen this door before. It's that second door on the first floor.
Note: When you reach the cutting blades, make a running jump, turning left in midair, and jump over all of the knives. You can do it without damage, but watch out - if you hit the wall, you'll probably die. This is the most difficult part of the level. When you reach the door, there is an possible, but not necessary, epic fight #7.

POSSIBLE, BUT NOT NECESSARY, EPIC FIGHT #7
I advise you to ignore this one, grab the detonator key and get outta here! You remember what I said about the door... Anyway, if you wanna be stuck, perform this fight, occurring only if you haven't killed the dog and the gunman earlier - through the locked door (check the EPIC FIGHT #2).

M16 ROOM
Get the key and out of here. Also if you wanna get stuck (see above why), pick up the M16 - the worst gun in TR2. Why is it the worst? Because you mustn't stop running if you want to make continuous fire.

USING THE DETONATOR KEY+TOWER CLIMB
Return to the ledge in-between the blade traps, do a running jump over the two blades in front of you to the ledge with the "babe" decal on the floor and a candle on the wall. Stand on the middle of the babe facing the candle and press action to use the key (reposition more towards the middle of the tile tile if it doesn't work). There will be no animation, but the event will trigger, this allows you to climb the stone tower you are now inside. Continue up the steps until you reach a ledge you need to climb with action. Keep climbing; it is essential that you DO NOT JUMP UP STEPS HERE, as you can get trapped in the void, only climb using action. After climbing up a series of smaller ledges you will come across another taller ledge; have a strong weapon like the Uzi's ready and then shimmy to the far right before climbing. On the higher ledge to your left is a Dog and a Flamethrower; if you stay against the back wall on the lower ledge you should be able to kill them safely. After they're dead climb up and go through the open door in front of you (the black pit to the right is nothing).

*THE* EPIC FIGHT
You are now properly on the roof/battlements (note that you can't backtrack after going through the door as there's an invisible wall). There's an absurd amount of enemies here even by this level's earlier standards (although the sheer size of the area works against them), and it will take a long time to grind through them all, so it's hard to give specifics; instead I will do some bullet points of general strategies for getting through with minimal health use (use weapons as you see fit, because you don't really need them past this):

* The general layout of enemies is that there are a bunch on the battlements you start on, along with several in the lower area below, many of them seem to spawn later as you explore this area, so don't get too complacent. The most obvious threat is eight Opera House Miniboss enemies on raised ledges in the lower area, but there are a few more on the battlements too.
* Stick to the high ground until you're sure the area is clear, as it's easy to make space up there and there's lots of space to retreat
* You will notice enemies at a distance flicker a lot and outright disappear. This is actually a big advantage for you because invisible enemies are "frozen" and you can kill them while they can't do anything; lock on to an enemy and then back off until they disappear and you can attack while they have no way to retaliate. Note that, while doing this, enemies only die when they appear again, so you'll have to move in a some point to finish them (and against tougher enemies you'll have to guess when that point is). Once you have some space you can take out most of the Minibosses quite easily (but slowly) doing this.
* There are plenty of Flamethrowers too so watch out, but this area is so big that they don't pose much of a threat. There's technically also water on the bottom floor to save yourself if you get burned, but if it happens early you'll likely get overwhelmed soon after anyway.

(Note that looking off the battlements to the mountainous side areas you might see a switch off one side, I don't know how to reach it and checking the level file reveals it opens nothing of value so ignore it)

THE AFTERMATH
Once you're sure both the upper and lower levels are clear (or 99% clear) it's time to head down to the lower level; either lower yourself on one of the ledges the minibosses were on, then drop down, or jump into the water pool. Ignore the slope as it's a pointless risk. Be careful of the broken glass (which is why the slope is a bad idea) and the small hole dropping down to the main hall (it'll kill you so ignore it). The enemies on the lower floor drop various items, so there will plenty to collect down there if you've cleaned up (though it's not especially important anymore unless your health is low). There are two switches to press down here. For one, there is a Miniboss ledge that is slightly lower and climbable to the right side (facing it) of the slope . Once you are on the ledge turn left and jump across to the Miniboss ledge nearest the wall the slope is on; there is a switch on the wall here. Now turn around and look down; there is another switch against the slope over broken glass; drop down and carefully walk over to press it. These switches open the final area (accessed from the large starting room) and your way off the roof.

GETTING BACK TO THE START
You are now faced with the problem that you are stuck in this "pit" with no way back up. Actually there is one, on the back part of the left side (facing it) of the slope there is a completely invisible ladder texture that you can climb up; once in the general area press action against the slope walls and you should find it. Now you are back on the battlements, head over to the tower structure on the right side (from the top of the ladder). One of the switches below opened the door here which leads to the stone tower where EPIC FIGHT #3 took place. Head down and back to the large starting hall (no enemies should be around, but be vigilant just in case).

THE FINAL BATTLE AND END
The exit lies through the door directly behind where you started the level. Go through and you are outside. One last battle, and it's a breather in comparison to the rooftop one; there's a set of Minibosses and a Flamethrower. The thing is that you have a ton of space to exploit the targeting and AI, so take your time to grind this group down and don't blow it getting cocky now. The exit lies through the gated door at the back wall from where you entered; to open it, check the right hand walls for a dark ledge behind a tree you can climb up, then go forward to fall into a small pit with the switch on one of the walls. Time to leave! Your reward for all this is a busted stats screen.